This invention relates to digital telephone equipment employing pulse code modulation (PCM). More particularly, it relates to the control of transmission levels on telephone trunk circuits; and more particularly yet, to the control of transmission levels on the digital portion of an encorder-decoder apparatus, commonly referred to in the art as a "codec".
Telephone equipment employing pulse code modulation (PCM) is now well known. One important interface in digital telephone switching equipment is the interface between the analogue signals (which originate at a subscriber's telephone set) and the PCM digital signals employed in the digital switching equipment itself. Equipment for performing both the function of encoding the analogue signals into PCM digital signals and the function of decoding the PCM digital signals into analogue signals are commonly referred to as "codecs". The attenuation presented to the analogue side of these codecs by the associated wiring (e.g. trunk circuits) is variable due to differing lengths of wire, etc. In order to overcome this problem various methods have been employed to control the transmission levels on analogue telephone trunk circuits.
These various methods include the use of potentiometers, switches, plug-in resistor networks, screw-down pads, etc., in order to permit initial and periodic adjustment of transmission levels of such equipment as voice-frequency amplifiers, analogue or digital carrier equipment, and of simple metallic paths. In the preceding methods, the variation in signal level is always introduced in the analogue voice frequency path, even with the digital carrier equipment.
Some sample encoder-decoder devices of the prior art are depicted in the following patents: U.S. Pat. No. 3,877,028 dated Apr. 8, 1975 to R. M. Thomas; U.S. Pat. No. 3,883,864 dated May 13, 1975 to R. M. Thomas; and U.S. Pat. No. 3,906,488 dated Sept. 16, 1975 to R. E. Suarez-Gartner.